Interpretive Summary: The two main commercial types of wheat are durum (Triticum durum L., 2n=4x= 28) and common (T. aestivum L, 2n=6x=42.) wheat, the latter being the more widely grown. Wheat has three growth habits, namely winter (wheats grown over the winter months that require vernalization and can withstand prolonged periods of below freezing temperatures; e.g. Fowler et al. 1999, Mahfoozi et al., 2001,), facultative (wheats grown over the winter months in mild climates which may or may not require vernalization and cannot withstand prolonged periods of below freezing temperatures), and spring (wheats grown during the spring and summer months that normally do not require vernalization and cannot withstand moderate periods of below freezing temperatures). Growth habit should be viewed as a continuum from winter wheat to facultative wheat to spring wheat. Because wheat can be grown through the winter or summer, is very drought tolerant, and is used primarily for human consumption, wheat is the most widely grown crop in the world (217,500,000 ha; http://www.nass.usda.gov/Publications/Ag_Statistics/agr06/CHAP01.PDF verified Sept. 17, 2006). In this chapter, we will concentrate on winter wheat breeding using examples from the United States programs with which we are most familiar. However, global winter wheat breeding will be discussed as appropriate. It should be noted that as a winter annual, winter wheat it is often grown in rotation with summer annual crops if the growing season is long enough and moisture is sufficient or there is supplemental irrigation.

Technical Abstract:
The two main commercial types of wheat are durum (Triticum durum L., 2n=4x= 28) and common (T. aestivum L, 2n=6x=42.) wheat, the latter being the more widely grown. Wheat has three growth habits, namely winter (wheats grown over the winter months that require vernalization and can withstand prolonged periods of below freezing temperatures), facultative (wheats grown over the winter months in mild climates which may or may not require vernalization and cannot withstand prolonged periods of below freezing temperatures), and spring (wheats grown during the spring and summer months that normally do not require vernalization and cannot withstand moderate periods of below freezing temperatures). Growth habit should be viewed as a continuum from winter wheat to facultative wheat to spring wheat. Because wheat can be grown through the winter or summer, is very drought tolerant, and is used primarily for human consumption, wheat is the most widely grown crop in the world (217,500,000 ha). In this chapter, we will concentrate on winter wheat breeding using examples from the United States programs with which we are most familiar. However, global winter wheat breeding will be discussed as appropriate. It should be noted that as a winter annual, winter wheat it is often grown in rotation with summer annual crops if the growing season is long enough and moisture is sufficient or there is supplemental irrigation.